A wide variety of manufactured goods contain processed sheet metal. For example, aircraft, automobiles, file cabinets and household appliances, to name only a few, contain sheet metal. The sheet metal is typically purchased directly from steel mills and/or steel service centers, but may be passed through intermediate processors (sometimes referred to as "toll" processors) before it is received by an original equipment manufacturer.
Various methods exist for flattening sheet metal and for conditioning the surfaces thereof. Flatness of sheet metal is important because virtually all stamping and blanking operations require a flat sheet. Also, in certain applications, such as in the aerospace industry, residual stress free material is critical. Good surface conditions are also important, especially in applications where the top and/or bottom surfaces of the metal sheet will be painted.
There are a number of common defects that effect sheet metal flatness. For example, when sheet metal is rolled into coil form for convenient storage and transportation, the strip takes on a coiled shape. This curvature is commonly referred to as "coil set." Coil set occurs because the sheet metal has been bent past its yield point. More specifically, when sheet metal is coiled, the metal fibers near the inside surface of the curved sheet are compressed past their yield point, and the metal fibers near the outside surface of the curved sheet are stretched past their yield point. Another type of shape defect known as "edge wave" occurs if the edge portions of the sheet are longer than the center portion of the sheet, resulting in undulations in one or both of the edge portions of the sheet. A similar type of shape defect known as "center buckle" results if the center portion of the sheet is longer than one or both of the edge portions, which results in bulging or undulating of the central portion of the sheet.
One method of removing coil set in sheet metal is "straightening." A conventional straightener is shown schematically in FIG. 1. In a straightener process, a strip of sheet metal S is advanced through a series of large diameter upper rollers U and lower rollers L, which are positioned relative to one another to put deep upward and downward bends in the sheet sufficient to reverse the coil set. However, straightening can only remove coil set and some cross bow. It is a rather crude and imprecise method that is typically used only as a first pass.
Another conventional method of flattening sheet metal is "roller leveling." A conventional roller leveler, shown schematically in FIG. 2, comprises a top set of small diameter rollers T and a bottom set of small diameter rollers B mounted in a frame (not shown) so that top and bottom sets of rollers are offset from one another. A series of larger diameter "back-up" rollers R engage the small diameter rollers T and B and can be adjusted as needed to flatten the material moving through the top and bottom rollers T and B. A strip of sheet metal S is advanced between the top and bottom sets of small diameter rollers T and B and is alternately flexed upwardly and downwardly between the top and bottom rollers such that the amount of flexing decreases as the sheet travels toward the exit end E of the roller leveler. The small diameter rollers T and B work the sheet S by bending the metal fibers near the inside surface of the curve and the metal fibers near the outside of the curve past their yield point (i.e., beyond their elastic limit). A roller leveler produces a reasonably flat metal sheet, but is extremely difficult to operate and requires a highly skilled operator. Moreover, the roller leveling process itself is less than ideal because there still exists a neutral axis in the sheet metal where the yield point of the metal has not been exceeded by the small diameter rollers. Metal fibers lying at or near this neutral axis may be in a stressed condition (and tend to spring back toward their original shape) because they have not been deformed past their elastic limit. Therefore, even after roller leveling, the material at or near the neutral axis will possess internal residual stresses because the grain structure is not uniform. Also, roller leveling alone does nothing to remove scale and corrosion from the surface of the sheet metal.
Another method of flattening sheet metal is "temper passing." A conventional "2-high" temper mill cut-to-length line is shown schematically in FIG. 3, along with a roller leveler L and a shearing machine M. The "2-high" temper mill comprises two large diameter rollers D that significantly compress the metal fibers at the top and bottom of the metal sheet S into uniformity. This results in a substantial reduction of internal residual stresses at the top and bottom surfaces of the metal sheet, but typically does not work the fibers near the neutral axis of the metal sheet past their yield point. Therefore, even after a 2-high temper passing process, the material at or near the neutral axis may still possess internal residual stresses and the material may not be sufficiently flat. A 2-high temper passing process does little to remove scale and corrosion from the surface of the sheet metal. In fact, because of the substantial compressive forces applied to the top and bottom surfaces of the sheet metal by the temper passing rolls, surface scale tends to become embedded in the metal surface, which can increase the likelihood of point source corrosion and consequent rusting.
"Temper passing" can also be accomplished with "4-high" temper mill (not shown), which comprises two upper rolls (an upper sheet engaging roll and a back up roll therefor) and two lower rolls (a lower sheet engaging roll and a back up roll therefor) all generally aligned in a vertical plane. The two sheet engaging rolls are much smaller in diameter than the rollers D of a "2-high" temper mill. As such, the two sheet engaging rolls of the "4-high" temper mill apply a more concentrated force at the point of contact. Like the "2-high" temper mill, the "4-high" temper mill significantly compress the metal fibers at the top and bottom of the metal sheet into uniformity, resulting in a substantial reduction of internal residual stresses at the top and bottom surfaces of the metal sheet. However, like the "2-high" temper mill, the "4-high" temper mill also fails to work the fibers near the neutral axis of the metal sheet past their yield point. "4-high" temper mills tend to do a better job of removing scale and corrosion than "2-high" temper mills.
In some applications, a certain amount of crown (i.e., thicker gauge in the center than at the ends) may be desired. A problem with both "2-high" and "4-high" temper mills is crown reduction (known as "feathering") in which the crown in the metal sheet is compressed out by the temper mill's rollers.
Another method of flattening sheet metal is "stretcher leveling." A conventional C-frame stretcher leveler is shown schematically in FIG. 4. Stretcher leveling is generally considered to be a superior flattening process because, unlike roller leveling and temper processing, it rectifies the problem of internal residual stresses and produces a flatter product without crown reduction. As shown in FIG. 4, a typical C-frame stretcher leveler includes a pair of generally C-shaped grippers or jaws G that securely grip the opposing ends of the sheet S to be stretched. The surface portions of the grippers that engage or grip the sheet metal to hold the sheet against movement during stretching are typically grooved, knurled or serrated to provide a secure grip. In operation, the grippers G are hydraulically or pneumatically controlled to engage the opposed ends of the sheet S and, once a firm contact is made, hydraulic actuators (not shown) move the grippers in opposite directions from one another to stretch the metal sheet S held therebetween. The entire cross section of the metal sheet is stretched past its yield point (i.e., beyond its elastic limit) such that all internal residual stresses are eliminated from top to bottom and from side to side. However, a problem with a conventional C-frame stretcher leveler is that it cannot be used with continuous strips of metal because the C-shaped grippers clamp at the opposed ends of a metal sheet, as shown in FIG. 4. Another problem with conventional C-frame stretcher levelers is that the grippers bite deeply into the metal and disfigure the top and bottom surfaces of the sheet. Traditionally, the disfigured portions of the sheet are cut off as scrap, which results in a substantial amount of wasted material. Also, operation of a C-frame stretcher leveler is very labor intensive because the individual sheets must be moved into and out of the machine between operations. Aside from cutting off the disfigured portions, conventional C-frame stretcher levelers do nothing to improve the surface quality of the sheet metal.
U.S. Pat. No. 4,751,838, issued to Kenneth Voges, discloses an "in-line stretcher leveler." The teachings of this patent are incorporated herein by reference. The basic components of conventional in-line stretcher leveler are shown schematically in FIG. 5. As shown in FIG. 5, a typical in-line stretcher leveler includes a first set of upper and lower gripping members G.sub.U1 and G.sub.L1 and a second set of upper and lower gripping members G.sub.U2 and G.sub.L2. The gripping members are hydraulically or pneumatically controlled to engage top and bottom surfaces of the metal sheet S and, once a firm contact is made, hydraulic actuators (not shown) move the first and second sets of gripping members in opposite directions from one another to stretch the segment of the metal sheet S positioned between the two pairs of gripping members. Then, the gripping members are released and the metal sheet S is advanced so that the next section of the metal sheet can be stretched. Because the gripping members of the in-line stretcher leveler engage the metal sheet from the top and bottom, rather than at opposing ends, the in-line stretcher leveler can be used to stretch any length of sheet metal by successive stretching operations wherein the metal sheet is successively advanced through the stretcher leveler after each stretching operation. As disclosed in U.S. Pat. No. 4,751,838, unlike the grippers of the C-frame stretcher leveler, the gripping members G.sub.U1, G.sub.L1, G.sub.U2 and G.sub.L2 of the in-line stretcher leveler preferably have engagement surfaces that are sufficiently smooth to avoid marring or otherwise disfiguring the surfaces of the metal sheet S. This is particularly advantageous because there are no disfigured portions to be cut off as scrap, which results in substantial cost savings. Also, this process is far less labor intensive than C-frame stretcher leveling. However, the in-line stretcher leveler disclosed in U.S. Pat. No. 4,751,838 does nothing to improve the surface quality of the sheet metal.
One known method of conditioning the surface of sheet metal is commonly referred to as "pickling." Pickling is a cleaning process used to remove black oxide scale and other smut that has formed on the sheet metal surface. In a typical pickling process, the metal sheet is run through a hydrochloric acid bath, a rinse tank, an air dryer, and an oiling station. The oil is applied to prevent corrosion of the bare metal surface. An advantage of pickling is that the sheet metal does not need to be perfectly flat, because the hydrochloric acid bath is typically deep enough to accommodate for some waviness caused by internal residual stresses. However, pickling does nothing to improve flatness.
Thus, there is a need for a sheet metal processing apparatus that incorporates the benefits of an in-line stretcher leveler together with a surface conditioning apparatus that removes scale and other smut from the surface in a continuous strip of sheet metal.